Time and Diurnal Motion

Transcription

1 Time and Diurnal Motion Time and Diurnal Motion A. Geography: mapping the earth 2 B. Equatorial Coordinates C. Local Horizon System Updated Sep 30, 2012 A. Geography: mapping the earth Geometry: measure the earth! 1) The earth is a ball 2) Geographic Coordinates 3) Definition of Time 3 1a. The Earth Is Round Pythagoras of Samos ( BC) the first pure mathematician lunar eclipses imply shadow of earth is round Bottoms of ships vanish first as ship goes to sea 4 1b. Aristotle ( BC) notes change in altitude of north star as travel south Estimates circumference of earth is 74,000 km (true value is 40,000 km) 5 1c. Eratosthenes ( BC) 236 BC Director Library of Alexandria 240 BC Was told that on midsummer day (June 21) in the town of Syene in southern Egypt (today Aswan, near a huge dam on the river Nile) the noontime Sun was reflected in a deep well, meaning that it was right overhead, at zenith. But at Alexandria, on same day it was NOT overhead! 6 1

2 Erastotenes In Alexandria sunlight was at angle of 7.2 degrees. Hired soldier to march out distance between cities: 5040 Stadia 7 2. Cartography Spherical coordinates probably first done by Pythagoras (470 BC), Aristotle (330 BC), Eratosthenes (240 BC) Hipparchus (120 BC). 8 Calculates circumference is 250,000 stades [within 1% true value] 2b. Latitude is measured up from equator Hayward is 37 N40 08 Equator is 0 N pole is 90 What is latitude Of South Pole? 9 2c. Longitude is measured westward from Greenwich, England Hayward is 122 W04 51 Greenwich is 0 What is the longitude of the north pole? 10 Greenwich Observatory England (established 1675) Longitude at Greenwich 2

3 13 3. Solar Time 14 Airy Transit Telescope 15 3a) Sundials measure solar time 3b) UT: Universal Time BC Berosus Sundial Shaped like a bowel. The longitude line on the bowel tells how many hours before or after noon the sun is. The transit of the sun (when sun crosses local prime merdian) happens at NOON (12 hours) local time. GMT: Greenwich mean time, the solar time at Greenwich Observatory [nearly the same as UT: Universal Time and ET: Ephemeris Time ] 3b) Time is Relative When its 6 pm here What time is it in Boston? 9 pm What time is it in Greenwich? 2 am Definition of time depends upon your longitude 17 3b) Time Zones are 15 (1 hr) wide 18 3

4 California is in Pacific Zone Pacific Standard time: PST PST = GMT 8 hours Pacific Daylight Time: PDT = PST c. Local Mean Time At Lake Tahoe (120 degrees longitude) sun transits on average at 12:00 noon PST. At Hayward, sun transits on average 12:08 PST. This is because we are 2 degrees longitude west of Lake Tahoe. It takes 8 minutes for the sun to travel from Lake Tahoe to here. 20 Local Mean Time: LMT = PST 8 minutes Sun Transits (on average) at 12:00 LMT 3c. Longitude Measure longitude in terms of time If 360 is equal to 24 hour 15 is equal to 1 hour 1 is equal to 4 minutes 1 is equal to 4 seconds Hayward s Longitude is hence 8 hours, 8 minutes, 19.4 seconds West of Greenwich, England 21 3c) Relate Time & Longitude LMT: Local mean time or sundial time the time at our location LMT=GMT - Longitude Hayward s Longitude is 8 h 8 m LMT =GMT 8 h 8 m LMT = PST 8 m 22 B1. Celestial Coordinates 23 Anaximander (580 BC) invents idea of celestial sphere. (?) Eudoxus (360 BC) makes early map of constellations Hipparchus (130 BC) made a star catalog of 850 stars with some sort of coordinates Claudius Ptolemy (150 A.D.?): The first really accurate map, 48 constellations, 1025 stars with measured ecliptic longitude & latitude Celestial 24 Latitude is Declination Celestial Longitude is Right Ascension Ascending Solar Node ( First Point of Aries ) defines 0 RA 4

5 B2) Diurnal Motion 25 2) Diurnal Motion 26 Earth Rotates on its axis every day Celestial Sphere appears to rotate around celestial pole Sky moves east to west You can mimic the moving sky with a starwheel The rivet is the north pole B3a). Star Time (sidereal) 27 B3a). Sidereal vs Solar Time 28 The RA (right ascension) which is transiting (crossing the prime meridian) is sidereal time Sundial (Solar) Time tells the position of the sun (noon or 12:00 = transit) Example: At 6 h Sidereal time, Betelgeuse (Orion) is transiting Sidereal (Star) Time: tells which stars are transiting Example, January 5 th at 3 am, the sidereal time is 10 hours (i.e. the star Regulus with RA=10 hours, is transiting) B3b. Sidereal Period 29 B3b. Sidereal Period (animation) 30 Solar day: 24 hours time between transits of sun Sidereal day: 23 hours 56 min time between transits of a star This means a star will transit 4 minutes earlier each day 2 hours earlier each month 5

6 C. Horizon Coordinates Altitude and Azimuth Your starwheel is a device for predicting the altitude and azimuth of stars, from the date and time 32 Arabian Astronomers perfected the Astrolabe, the ancestor of your starwheel (more properly called a planisphere Relate Horizon to Equatorial 34 Declination of zenith is your latitude Declination at North point is 90-Latitude (52 ) Declination at South point is Latitude-90 (-52 ) Stars inside the circle of perpetual occultation are called circumpolar, and never set! If you lived at the north pole, all stars would be circumpolar! Polaris would be at zenith 6

7 If you lived at the equator, no stars would be circumpolar! Polaris would be at horizon 37 Starwheel 38 shows what is visible As you turn the wheel, some things never vanish! 3. Path of a star 39 Will follow a parallel of declination Transit altitude is 90 - dec lat Rising/setting points will be same each day! 40 7